In Patent Literature 1, there is a description that, in an electric braking device using an electric motor, in order to “carry out a smooth swing motion of a piston during braking,” “a spherical surface is formed at a portion of a pressing member to be engaged with a piston, and grease is filled between the spherical surface and an abutment part of the piston; after the braking force for the wheel is released, the electric motor is rotated backward, to thereby move the nut in a direction away from the piston so that the pressing member is brought into a free state to form a clearance between the spherical surface and the abutment part” (refer to Abstract and the like of Patent Literature 1).
In Patent Literature 2, there is a description that, in an electric braking device using an electric motor, in order to “avoid, on a ball screw configured to convert a rotation of the motor into a translational motion to move brake pads, the absence of an oil film caused by a repeated movement in a specific extent, and an increase in a friction thereof,” “when a vehicle stops and a brake pedal is released, the brake pads for the respective wheels are simultaneously moved toward a pressing release side until a nut opposes a position beyond a usage section of a screw shaft in a normal brake operation, and are then returned to a neutral position; and the brake pads are sequentially moved to a pressing side for each wheel, and are then returned to the neutral position” (refer to Abstract and the like of Patent Literature 2).
This configuration is intended to maintain an appropriate lubrication state in a manner that “balls of the nut in contact with the section of the screw shaft other than the normal usage section are brought into contact with the normal usage section of the screw shaft, and hence the lubricant is supplied to the normal usage section of the screw shaft via the balls of the nut, to thereby recover the oil film in the normal usage section of the screw shaft.”
In Patent Literature 3, there is a description that, in order to “prevent extraneous substances from entering from the outside into a nut of a ball screw, and to prevent a lubricant in the nut from leaking to the outside,” “in a seal device, a pair of annular seals each attached on an end part of the nut of the ball screw and having an elastically deformable seal lip are arranged with a predetermined gap by an annular spacer, a seal lip inner diameter contour of the seal is formed into a shape similar to a vertical cross-sectional shape of the screw shaft, and has a diametrical dimension slightly smaller than an outer diameter of the screw shaft; and a space between both the seals on the inner diameter side of the spacer serves as a lubricant filling space” (refer to Abstract and the like of Patent Literature 3).
In the electric braking device using the electric motor disclosed in Patent Literature 1, lubrication of a so-called “universal joint mechanism (for example, a spherical surface and an abutment part)” as well as lubrication of “a rotation/translation conversion mechanism (for example, a screw member)” is important. Therefore, in the device disclosed in Patent Literature 2, the “ball screw” is employed as the rotation/translation conversion mechanism, and the operation of moving the brake pads beyond the normal usage range (so-called “retraction of ball screw”) is carried out, thereby adjusting the lubrication state of the screw shaft. However, in order to improve the lubrication state of the screw, in addition to the retraction operation, considerations need to be taken for the lubrication of the screw itself.
Patent Literature 3 describes the seal device for the ball screw used as an operation part for a moving device and a positioning device in a machine tool and the like. In those devices, maintenance using grease and the like may be periodically carried out. For the vehicle electric braking device, the lubrication state of the screw needs to be maintained longer than in the machine tool and the like.